Fast Failback; Sizing Replica Reserve And Delegated Space - Dell EqualLogic PS6100 series Configuration Manual

With replication, it does not matter if the volume is thin provisioned or uses a traditional volume since
in either case only the data that has changed will be copied to the replica. On the secondary side,
volumes are always thin provisioned to conserve available capacity used by the Replica Reserve for
that volume.
6.1.4

Fast failback

With fast failback enabled you can ensure that the volume data preserved by the failback snapshot on
the primary group always matches the volume data in the most recent replica stored on the secondary
group. If you have an event that causes failover to the secondary group and the workload
subsequently writes changes to the replica volume then failback snapshot supports a quicker fail back
to the primary group by replicating only the changes made to the replica volume during the time it
was active as a recovery volume on the secondary group. If the failback snapshot is not enabled, you
must replicate the entire volume contents back to the primary group to complete the failback
operation. Depending on the size of the volume, the failback scenario can take significantly longer to
complete if Fast Failback is not enabled.
6.1.5

Sizing Replica Reserve and Delegated Space

The volume replication process in EqualLogic PS Series storage consumes extra storage space on both
the primary and secondary group partners. In addition to the space consumed by the volume itself,
each volume will require additional space in the primary group for Replication Reserve and Local
Reserve, plus delegated space for storing replica sets in the secondary group. This is illustrated in
Figure 14 below. A single delegated space on the secondary group must be used for all volumes
received from a specific primary group.
Delegated space for a replication partnership must be assigned to a pool. Delegated space for
different replication partnerships can be assigned to different pools on the secondary group.
Note: Any type of application or operating system level process that causes block level
data changes will affect replication data size and time to complete. For example, file
system defragmentation of a large partition will significantly increase the amount of
space consumed in replica reserve, delegated space on the secondary group and time to
complete the next scheduled replication.
March 2013 Dell EqualLogic Configuration Guide v14.1 6-46